They're cheap because they're mass market items. A lot more people buy TV dongles to watch TV than they do to mess around with SDRs.

The hobbyist market for SDRs is tiny, and thus it would be very cost-ineffective to manufacture them for just this market. Instead we're taking something that already exists (TV tuners), is mass market, and by coincidence, just happens to be a software-defined radio, and re-purposing it.

A good analogy would be modifying an existing bicycle to be an electric bicycle, as opposed to manufacturing a dedicated electric bicycle from scratch. Sure, the latter will probably perform better and look nicer but it would be orders of magnitude more expensive, because the market for electric bicycles is small.

(99% of the electric bikes on the market are just modified mountain bikes)